I have an 88 Baja with 454's. If I wanna drop in a HP 500, what do I have to change?
Just the flywheel?

Sea pump too. Need one with fuel pump on it or an electric fuel pump.

There's no fuel pump on those? That sux!! I guess I could go electric, that's probably a better solution than a new sea water pump?
Is that year a gen V or VI?

Honestly I am not positive. If gen v or vi that year off the top of my head but neither of those have a block mounted fuel pump.

Gen 6, buy a complete motor with flywheel and all the accessories.

The 99 502 has no fuel pump boss but the latest gm casting does.

So what Gen and year is the latest casting? Also, if the fuel pump and flywheel is my only issue, I don't think that's a show-stopper. I may just go with the 99 HP 500.

Do a comparison of the two engines' parts diagrams on www.mercruiserparts.com. The HP 500's definitely require performance exhaust manifolds (MercuryGil were standard) in case the engine come w/o manifolds.